Pneumatic piston return system and valve assembly for impact tools

ABSTRACT

A piston return system for air cylinders in an impact tool wherein live air, when introduced into the cylinder on the pressure side of the piston, initiates the power stroke. A check valve by-passes live air around the cylinder to an expansion chamber on the opposite side of the piston where it expands when air pressure is relieved on the pressure side of the piston to return the piston to its uppermost position. The expansion air is bled to atmosphere through a passageway in the piston and through a valve carried by the piston when the piston returns to its uppermost position. A cartridge-type valve assembly is mounted in the tool to bleed air through a bore in one of the valve members to the atmosphere from the pressure side of the piston and from the expansion chamber when the piston reaches its uppermost position.

I United States Patent [151 3,653,299

Howard [4 1 Apr. 4, 1972 [54] PNEUMATIC PISTON RETURN SYSTEM 3,363,5171/1968 Powers ..9l/4l6 AND VALVE ASSEMBLY FOR IMPACT 3,427,928 2/1969Bode ..9l/308 V TOOLS Primary Examiner PaulE Maslousky [7 2] lnventor:Frank C. Howard, Mt. Prospect, lll. Attorney-Dressler, Goldsmith,Clement & Gordon [73] Assrgnee: SIgnode Corporation, Chicago, Ill. 57]ABSTRACT [22] filed: May 1970 A piston return system for air cylindersin an impact tool [21] Appl.No.: 36,313 wherein live air, whenintroduced into the cylinder on the pressure side of the piston,initiates the power stroke. A check valve by-passes live air around thecylinder to an expansion 2? chamber on the opposite side of the pistonwhere it expands i 91/416 461 5 308 220 when air pressure is relieved onthe pressure side of the piston 1 le 0 7 to return the piston to itsuppermost position. The expansion air is bled to atmosphere through apassageway in the piston and through a valve carried by the piston whenthe piston [56] References Cited returns to its uppermost position. Acartridge-type valve as- UNITED STATES PATENTS sembly is mounted in thetool to bleed air through a bore in one of the valve members to theatmosphere from the pressure 2,703,558 1955 side of the piston and fromthe expansion chamber when the 7 2, 8 1 piston reaches its uppermostposition. 3,27 ,l 4 l 1 3,313,213 4/1967 1] Claims, 3 Drawing FiguresPATENTED R SHEET 1 OF 2 6) MW M/M PNEUMATIC PISTON RETURN SYSTEM ANDVALVE ASSEMBLY FOR IMPACT TOOLS Y This application is an improvementupon the subject matter of copending U.S. applicationSer. No. 732,441,filed May 27, 1968, now U.S. Pat. No. 3,552,274. 1

The present invention relates to impact tools of the type employing apneumatically operable piston and cylinder 'assembly for powering thedriver ordinarily associated with'such tools. The inventionhasparticular reference to'an improved air return and valving system forreturning the piston of such an assembly to its retracted position afterthe :piston has delivered its power stroke.

Insofar as piston return means are concerned, pneumatically operablepiston and cylinder assemblies may, broadly,be divided into twogroups.'In one group the piston is returned by spring pressure and inthe other group the piston is returned by air pressure. It is to thislatter classv of piston and cylinder assemblies that the presentinvention pertains.

Numerous air return systems are currently employed for returning apiston to its retracted position within a cylinder. Certain of thesesystems rely upon the use of live air for driving the piston in bothdirections. Such systems possess the obvious disadvantages that a largevolume of air must be expended during each operated cycle. To obviatethis limitation, certain other air return systems have been devised inwhich all, or at least a limited portion, of the air which is expelledfrom the cylinder during the power stroke of the piston is col lected ina fixed pressure chamber and compressed therein by the compressiveaction of the piston during the power stroke. After the piston hascompleted its power stroke, this stored and compressed air is returnedto the cylinder and caused, by expansion thereof, to return the pistonto its retracted position. While such systems conserve an appreciableamount of compressed air, the placement of the pressure chamber entailsdifficulty in cylinder design. Placement of the pressure chamber at theend of the cylinder on the driver side of the piston requires a cylinderof undue length, while placement of the pressure chamber at one side ofthe cylinder increases the overall bulk of the tool. In either event,the use of offset pressure chambers, regardless of their location withrespect to the cylinder, consumes a space which must be sufficientlylarge as to enclose a volume of air under pressure which, when releasedinto the cylinder on the driver side of the piston, will maintain aneffective expansion force sufficient to completely return the piston forthe next succeeding power stroke thereof.

The present invention is designed to overcome the abovenoted limitationsthat are attendant upon the construction and operation of conventionalpiston return systems of the air return type, and is an improvement uponthe piston return system described and claimed in a copendingapplication Ser. No. 732,441, filed on May 27, 1968, now U.S. Pat. No.3,552,274. The invention contemplates the provision of an improved airreturn system wherein that portion of the cylinder chamber which isdisposed on the driver side of the piston at the time the latter is atthe end of its power stroke is utilized as an expansion chamber intowhich live air is injected for piston return purposes. The live air isobtained by the expedient of by-passing air from the pressure side ofthe piston between the piston cylinder and the body portion of the toolsurrounding the piston cylinder and past a sealing ring which is ofspecial construction and functions as a check valve for theunidirectional passage of live air into the expansion chamber. When thepower stroke is completed the air in the expansion chamber drives thepiston to its uppermost position and the return air is then bled toatmosphere directly through the piston and through an improved cartridgevalve assembly.

In accordance with this invention, the piston is provided with apassageway which is in communication with the expansion chamber. Thispassageway is closed by an annular sealing ring which is automaticallyopenable in response to the pressure of the return air when the pistonreturns to its uppermost position. At this time, an annular sealing ringmounted on the piston expands sufficiently to allow the return air topass through the passagewayand to be bled from above the piston. Inaddition to this annularring, the piston mounts a further annularsealing ring which sealingly engages a cylinder wall during the entirerange of movement of the piston. This annular ring also serves to holdthe piston in its up position after it is returned to that position forthe next power stroke by'the retumair.

Further, in accordance with this invention, an improved valving systemis provided in which a valve assembly is manufactured as a cartridgewhich may be inserted (and removed) as a'unit from the impact toolcasing. This cartridge, besides a mounting sleeve for mounting thecartridge in the casing, comprises a pair of valve-members, one of whichis disposed in a bore in the other. The valve member defining the boreis moveable between a pair ofpositionslln the'first of those positions,the pressure chamber above the piston is out of communication with thesource of pressure air, but is in communication with the bore of thevalve member and therethrough with the air exhaust chamber which leadsto the atmosphere. In the other position of that valve member, thereceived valve member sealingly closes the bore to prevent communicationwith the atmosphere and to provide communication between the pressurechamber and the source of live or pressure air.

The improved piston return system and valving system of the presentinvention has been designed for use primarily in portable impact toolssuch as magazine fed stapling and nailing machines, and other deviceswhich rely for their action upon the pressure stroke of a pneumaticallyoperated driver. It is to be understood, however, that the invention isnot necessarily limited to such use, and that piston and cylinderassemblies and valving assemblies constructed in accordance with theprinciples of the invention may be used in other environments as well.

The provision of an impact tool which is extremely simple in itsconstruction and which therefore may be manufactured at a low cost, onewhich is comprised of a minimum number of parts, one which is rugged anddurable and which therefore will withstand rough usage; one which iscapable of ease of as sembly and disassembly, and one which is welladapted to perform the service required of it, are further desirablefeatures provided by the present invention.

Further features, objects and advantages of the present invention willbecome apparent from the following description and drawings of anillustrative embodiment of the invention of which:

FIG. I is a side elevational view, partially in section, of a pneumaticstapling tool embodying the principles of this invention;

FIG. 2 is an enlarged view of a portion of FIG. 1 illustrating the valveassembly in a piston driving position; and

FIG. 3 is a fragmentary cross-sectional view of a portion of FIG. 1.

Referring now to the drawings in detail, the piston return and valvingsystems of the present invention are shown, for exemplary purposes, asbeing operatively embodied in a portable guntype percussion or impacttool in the form of a pneumatically operated, magazine-fed stapling tool10. The stapling tool comprises several principal parts, including acasing comprising a cylinder-enclosing body portion 12 from which thereprojects sidewardly an integral lateral extension or handle portion 14,a closure head 16, a nosepiece 18, a staple magazine 20, and a valvingassembly 22.

The body portion 12 is of generally tubular construction, the upper endthereof being open while the lower end is closed by means of arelatively thick bottom wall 24. The tubular body portion 12 surroundsand encloses a thin-walled sleeve like cylinder 26 within which there ismounted for vertical reciprocation a piston 28 which carries a driver 30for staples S disposed in the staple magazine 20. The upper end of thedriver is secured by means of a pin 32 in a slot fonned in the piston,and the lower end is guided in a slot 34 formed in the nosepiece 18. Thelower rim portion of the body portion 12 is provided with an annularelastomeric bumper pad 36 designed for engagement with the piston 28when the latter completes its downward power stroke. An elastomeric seal38 for the driver 30 is centered within the annular bumper pad toprevent egress of air from the lower regions of the body portion 12.

The magazine may be of any conventional type. In the illustrated form ofmagazine, the staples S are supported in straddling relation on a guidebar 40 disposed within a magazine chamber and are urged forwardly in themagazine toward slot 34 by means of the usual spring-pressed follower(not shown). The forward upper edge of the bar 40 constitutes a shearingedge by means of which the leading staple S in the staple cartridge maybe sheared from the cartridge each time it is engaged by the driver 30.

The upper circular rim of the cylinder 26 terminates flush with theupper rim of the tubular body portion 12 and is provided with athickened flange 44 which is formed with a continuous annular groove 46therein for reception of a specially configured sealing O-ring 48 whichserves as a unidirectional check valve and by means of which thecylinder 26 is sealed to the inner wall surface of the body portion 12for a purpose to be described. The lower edge of the cylinder 26 restson a segmental support shelf in the body portion to prevent downwardaxial movement with respect to the body portion 12.

The wall of cylinder 26 is spaced slightly inwardly from the wall ofbody portion 12, the two walls being generally coaxial so that theydefine therebetween a generally annular chamber 50 which is fullcylinder height. A series of ports 52, which may be four in number, areprovided by the shelf supporting the lower edge of cylinder 26, andestablish communication between the interior of the cylinder 26 and thechamber 50. The ports 52 are preferably equally and circumferentiallyspaced around the cylinder wall and are below the lowermost level towhich the piston 28 moves.

The portion of the cylinder chamber below the piston constitutes avariable volume expansion chamber 54 while the annular chamber 50constitutes an auxiliary fixed volume expansion chamber, the function ofwhich is to increase the capacity of the chamber 54 so that a greateramount of compressed air will be available for expansion purposes forthe return stroke of piston 28.

Piston 28 is generally of cylindrical design and the side wall thereofis provided with a pair of annular grooves 56, 58 in the upper regionthereof, and within which there are disposed annular gaskets or sealingrings such as O-rings 60, 62, respectively. During reciprocation of thepiston, sealing ring 60 functions to seal the piston 28 against the wallof cylinder 26, on the power stroke, and to prevent reverse flow of airaround the piston during the return stroke of the piston under theinfluence of expanding air within the expansion chambers 50 v and 54.The sealing rings 60, 62 are formed of a suitable elastomeric materialsuch as rubber, either natural or synthetic, or a rubber substitute.

Sealing ring 60 is always in sealing engagement with the wall ofcylinder 26. However, sealing ring 62 may be in contact with the walledcylinder 26 at all times except when the piston 28 is substantially inthe position of FIG. 1, i.e., the uppermost position, at which time thereturn air in chambers 50 and 54 will break the seal between the O-ring60 and air vent apertures 64 to allow return air to be vented fromexpansion chambers 50 and 54. Air vent apertures 64 are in communicationwith chambers 50 and 54 via a radial bore 66'which in turn communicateswith an axial bore 68 which terminates at the end of piston 28 in a ventopening 70. It is clear that pin 32 is suitably apertured so that itwill not block axial bore 68.

Preferably O-ring 62 is stretched slightly into groove 58. In that caseit may not sealingly engage the wall of cylinder 26. If it is stretchedtoo tightly, however, it will impede the exhaust ing of expansion airfrom chambers 50 and 54. On the other hand, if it is too loose andtightly seals against the cylinder, on

its repeated movement past the shoulder 69 of the relieved area 71 ofcylinder 26, it will be gradually abraded away until it fails tosatisfactorily perform its function of preventing the escape ofexpansion air prematurely.

It will be seen, therefore, that return air cannot and will not passinto apertures 64 from above the piston because of the sealing fitbetween O-ring 62 and vent apertures 64. However, when the pistonreturns to the position of FIG. 1, the relieved section 71 of thecylinder 26 will allow the expansion air to stretch the ring 62sufficiently outwardly to permit its escape through apertures 64 andupwardly to be vented from under the closure head in the manner to bedescribed.

The closure head 16 is secured in any suitable or known manner (as forexample by fastening bolts, not shown) to the upper open end of the bodyportion 12. A rigidified gasket means 72 is interposed between theclosure head and the upper rim of the body portion in sealingrelationship. The outer end of the integral hollow handle portion 14 isadapted to be connected by a suitable fitting 74 to a flexible conduit76 leading to a source of air under pressure which is adapted to beplaced in communication with a pressure chamber 80 disposed above theupper rim of the body portion 12 and below closure head 16. Pressurechamber 80 provides air to the piston 28 for its power stroke and toexpansion chambers 50 and 54 through a gasket port 82. When air from thesupply chamber 92 in the handle is admitted to pressure chamber 80, inthe manner to be described, the air thus serves both to drive piston 28and to fill the air return or expansion chambers 50, 54 via sealing ringcheck valve 48.

Sealing ring 48 is of the check valve type described and illustrated incopending application Ser. No. 732,441, filed on May 27, 1968, now U.S.Pat. No. 3,552,274, and operates in the manner described therein. Thering is provided with a series of transverse grooves 94 at its loweredge which provide air flow passages for the downward flow of airbetween the wall of body portion 12 and cylinder 26 from chamber 80 tofill expansion chambers 50 and 54 when driving air is admitted topressure chamber 80. Air present in chamber 54 is compressed into airstorage chamber 50 during the downstroke of the piston 28. On the returnor upstroke of piston 28, ring 48 sealingly engages the wall of bodyportion 12 substantially to prevent the leakage of return air therepast.As such, it is seen that O-ring 48 acts as a unidirectional check valve,generally in the manner described in U.S. Pat. No. 3,552,274, and forexample as O-ring 270 illustrated in FIGS. 9-11 therein.

Air for driving the piston is supplied to pressure chamber 80 (and toexpansion chambers 50 and 54) from chamber 92 through the valvingassembly 22 of this invention. Venting also takes place through valvingassembly 22. Valving assembly is a cartridge type unit and is supportedand positioned adjacent body portion 12 in a suitable stepped bore 102formed in the casing of the stapling tool 10.

The cartridge is surrounded for part of its axial length by a generallycylindrical sleeve 104 mounting a sealing ring 106 which is seated inbore 102. Sealing ring 106 sealingly engages bore 102. Sleeve 104defines a chordal notch 108, which is adapted to be keyed to the bore102 to prevent axial movement, and a chordal slot 110 adjacent its base.The keying of sleeve 104 to the bore is accomplished by driving a pin,such as a roll pin 109, through a bore passing through the sidewalls ofthe casing and into stepped bore 102. Pin 109 rests in confronting notchsegments 111 and 113 (FIG. 3). Chordal slot 110 provides communicationwith a vent passage 112 which opens to the atmosphere at the end remotefrom valve assembly 22. A spool valve member 114 is coaxially mounted insleeve 104 to move from the position shown in FIG. 1 to that shown inFIG. 2. In FIG. 2, pressure chamber 80 and supply chamber 92 are incommunication via pressure port 116 to drive the piston and to fill theexpansion chambers 50, 54 with return air.

In the closed position of P16. 1 pressure port 116 is shut off fromcommunication with the pressure air by virtue of the sealing engagementbetween annular flange 118 and gasket 72. Flange 118 is maintained inits up position by the high pressure air acting on thelower surface 120of the spool valve member, and by the compression spring 121 actingupwardly against lower surface 120. The sum of these forces exceed thedownward force exerted against surface 122 of the spool valve member,hence the normal position of the spool valve member 114 is the upper orclosed position of FIG. 1. Compression spring 121 serves an additionalfunction, acting as a safety to prevent firing when tool is firstconnected to a pressure air supply. The force exerted by the spring issufiicient to assure the maintenance of the closed position of FIG. 1 atall times, save that in which the trigger serves to actuate the tool.

High pressure air (from chamber 92) is normally provided to lowersurface 120 through an axial bore 124 which extends through spool valve114 from surface 122 to an annular space 126. Annular space 126 isdefined by the main body portion 128 of the spool valve member 114 and acomplementarily threaded fitment 130 (which defines lower surface 120)sealingly secured thereto by a suitable gasket 132, and by theconcentric, internally received lower valve member 134. Lower valvemember 134 defines a pair of diametric ports 136 which are incommunication with space 126 and which open inwardly of spring chamber138 in which resilient ball valve 140 is positioned. When the resilientball valve 140 is in the down position of FIG. 1, pressure air will passfrom spring chamber 138 over the upper surface of ball valve 140 (andbelow the upper concave ball seat 142), through ports 144, thence belowlower surface 120. In that manner pressure air is normally provided tolower surface 120 to maintain the spool valve member in its upperposition of FIG. 1.

It will also be apparent that the high pressure air acting on surface120 will act against gasket 146 to urge it downwardly against the end148 of the stepped bore 102. Because lower valve member 134 provides ashoulder 150 on which the inner edge of gasket 146 rests, the highpressure air will maintain the lower valve member in the fixed lowermostposition of FIG. 1.

Spool valve member 114 is moveable axially relative to the concentriclower valve member 134 to provide air to pressure chamber 80. Thismovement is initiated by the pivotal movement of a trigger 152 from theposition of FIG. 1 (the rest position), to that of FIG. 2, the powerstroke position. Trigger 152 elevates trigger rod 154 which lifts ballvalve 140 from the concave lower seat 156 into sealing engagement withthe upper concave valve seat 142; That results in the closing off of thesupply of high pressure air to lower surface 120 creating an imbalancein the forces maintaining spool valve member 114 in its upper position.That is because annular surface 122 has a sufficiently greater area thanflange surface 118 and the pressure exerted against it is also to offsetthe upward biasing force of compression spring 121. As such, the highpressure air drives the spool valve member downwardly toward and againstthe relatively fixed valve member 134, and the spool valve memberimmediately, therefore, moves to the position of FIG. 2.

It should be noted that as spool valve member moves to the position ofFIG. 2, it is guided in that movement by a plurality of annular sealingrings R, such as O-rings, carried by it and by the lower valve member134. These sealing rings, in addition to guiding the spool valve memberin its movement, serve to sealingly engage the confronting cylindricalsurfaces along which they move.

When the spool valve member moves to the position of FIG. 2 severalthings occur. One is that high pressure air from chamber 92 is admittedto pressure chamber 80 to begin the power stroke to drive piston 28 toits lowermost position, and to fill expansion chambers 50 and 54.Another is that the vent passage 112 is closed from communication withthe exhaust bore 160 in spool valve member 114. As seen in FIG. 2, aconical surface 164 of exhaust bore 160 is placed in sealing engagementwith a sealing ring 162 carried by lower valve member 134. This preventsthe passage of high pressure air downwardly through exhaust bore 160. Ofcourse, high pressure air will continue to act upon annular surface 122as well as upon the uppermost surface of lower valve member 134, therebymaintaining the lower valve member in its position in the end of steppedbore 102.

The piston 28 will remain in its lowermost position, after having drivena staple, and spool valve member 114 will remain in its lowermostposition (that of FIG. 2), until trigger 152 is released. Upon itsrelease, trigger lift rod 154 will be biased downwardly by compressionspring 170 through ball valve 140. Air will then pass below the concaveupper ball valve seat 142 and will act against lower surface 120 tocause spool valve member 114 to rise to the position at which annularflange 118 is sealed against gasket 72. That will cut ofi' communicationof the pressure air with pressure chamber at which time the compressedair stored in chambers 50 and 54 will expand to return piston 28 to itsuppermost position. The high pressure air trapped in pressure chamber 80at the time annular flange 118 moves into sealing engagement with gasket72 will be vented and bled through exhaust bore 160 past conical surface164 through a plurality of vent ports 172 into annular chamber 174downwardly through chordal notch and into vent passage 112. When thepiston returns to its full up position, that of FIG. 1, the return orexpansion air will stretch or expand O-ring 62 sufficiently to permitthe return air to be bled through axial bore 68, radial bore 66, ventapertures 64, gasket port 82 and then into exhaust bore 160, thence intothe vent passage 112 in the same manner as the pressure air trapped inpressure chamber 80 passed into the vent passage 112.

It will be thus be seen that the valve system of this invention providesa highly advantageous cartridge valve assembly which may be dropped as aunit into, and removed as a unit from a suitable bore provided in astapler tool or an impact tool casing. The valve assembly comprises twovalve members and a sleeve, one of the valve members defining an axialbore and being axially moveable, the other being disposed in said bore.The valve assembly serve both to supply pressure air to the drivingpiston for the impact or power stroke and to vent the exhaust airdirectly through the valve to the atmosphere. The exhaust air thus maybe bled from the tool at the rearward end, rather than through the headas is a common practice.

Further, a highly efficient and effective air return system for thepiston is also provided. The piston is provided with a pair of annulargaskets or O-rings one of which sealingly engages the cylinder andassists in holding the piston in an up position for driving, while theother stabilizes the piston while also providing an efficient andeffective means for venting the air return expansion chambers to theatmosphere through the valve assembly.

What is claimed is:

1. In a pneumatically operable impact tool comprising a casing having agenerally tubular body portion, and a reciprocable piston mountedtherein to be driven and returned pneumatically, said casing defining achamber containing high pressure air, and means for supplying air tosaid high pressure chamber, a pressure chamber above said piston forsupplying driving air to drive said piston downwardly, an expansionchamber effectively below said piston for returning said piston to anupper rest position, and an air vent passage defined by said casing forventing air to the atmosphere, an improved valve assembly mounted insaid casing and comprising a first valve member and a second coaxialvalve member, said valve members being relatively axially moveable, acommunication port between said high pressure chamber and said pressurechamber, said first valve member being moveable from a first position inwhich it sealingly blocks said communication port, and a second positionin which it opens said communication port, said first valve memberdefining a bore therein which is in constant communication with saidpressure chamber in both of said positions, said second valve memberbeing disposed in said bore, and in said first position providingcommunication between said bore and said air vent passage, andcooperable means on said valve members for sealingly preventing suchcommunication in said second position, means for normally maintainingsaid first valve member in said first position whereby said pressurechamber is normally in communication with the atmosphere through saidbore, and

means for effecting the movement of said first valve member to saidsecond position.

2. In the pneumatically operable impact tool of claim 1 in which saidsecond valve member mounts a ball valve and said means for effectingmovement includes a trigger rod for moving said ball valve from a firstball valve seat to a second ball valve seat.

3. In the pneumatically operable impact tool of claim 2 in which saidhigh pressure air chamber is in constant communication with said ballvalve through a second bore in said first valve member and the highpressure air maintains said first valve member in said first positionwhen said ball valve is on said first ball valve seat.

4. In the pneumatically operable impact tool of claim 1 in which saidcooperable means on said valve members comprises a coaxial seat and asealing ring which are out of engagement in said first position andwhich are in sealing engagement to block said bore in said secondposition.

5. In the pneumatically operable impact tool of claim 1 in which saidvalve assembly includes a sleeve surrounding said first valve member,said sleeve being fixed against axial movement in said casing, saidfirst valve member sealingly engaging said sleeve and being moveabletherein, said first valve member and said sleeve defining a chamberwhich is in fiow communication with said air vent passage and said borein said first position.

6. In the pneumatically operable impact tool of claim 1 in which saidsecond valve member encloses a ball valve and mounts a resilientlyloaded trigger rod for moving said ball valve against said resilientload, and said second valve member defines a chamber above said ballvalve which communicates with said high pressure chamber through asecond bore defined by said first valve member.

7. In the pneumatically operable impact tool of claim 1 in which saidmeans for normally maintaining said first valve member in said firstposition comprises spring means resiliently biasing said first valvemember to said first position, said biasing means preventing driving bythe tool when it is first connected to high pressure air.

8. In a pneumatically operable device for repeatedly exerting a drivingwork force in one direction and having a casing, a tubular body portion,a sleeve-like cylinder in said body portion and, in combination with thewall of said body portion, defining a narrow fixed volume expansionchamber, a piston sealingly slidable in said cylinder between retractedand extended positions, a variable volume expansion chamber forwardly ofthe piston, a closure head for the rear end of the cylinder and defininga pressure chamber rearwardly of the piston, said expansion chambersbeing in flow communication in all positions of the piston, valve meansselectively operable to admit live air at working pressure to saidpressure chamber to drive the piston forwardly to its extended positionand to relieve such pressure to allow the piston to return to itsretracted position under the influence of the expansion of air in saidexpansion chambers, and one-way check valve means automaticallyeffective during forward movement of the piston and after the piston hasassumed its full forward position for admitting live air at workingpressure directly into said expansion chambers from said pressurechamber between said tubular body portion and said cylinder, said checkvalve means being effective to seal said expansion chambers againstescape of entrapped air to said pressure chamber during substantiallythe entire return stroke of the piston, the improvement comprising valvemeans carried by said piston automatically openable by the expansion airin said expansion chambers when said piston has returned substantiallyto its rearwardmost retracted position for venting said expansion airthrough said piston to the pressure chamber and to the atmosphere 9. Inthe pneumatically operable device of claim 8 in which said pistondefines a passageway in constant communication with said expansionchambers and in communication with said valve means, and in which saidvalve means is biased to a position in which said passageway issealingly closed thereby, said cylinder being relieved adjacent therearwardmost position of the piston, at which position the valve meansis automatically opened by the expansion air in said expansion chambers,thereby to vent said air to atmosphere.

10. In the pneumatically operable device of claim 9 in which said valvemeans is an annular gasket and said passageway includes at least oneport sealed by said annular gasket, said annular gasket being biased toan open position automatically adjacent the relieved portion of saidcylinder.

1 1. In the pneumatically operable device of claim 9 in which said valvemeans is an annular gasket mounted in an annular groove defined by saidpiston, said annular gasket being stretched into sealing engagement withthe inner wall of said annular groove, said annular groove defining portmeans communicating with said passageway.

1. In a pneumatically operable impact tool comprising a casing having agenerally tubular body portion, and a reciprocable piston mountedtherein to be driven and returned pneumatically, said casing defining achamber containing high pressure air, and means for supplying air tosaid high pressure chamber, a pressure chamber above said piston forsupplying driving air to drive said piston downwardly, an expansionchamber effectively below said piston for returning said piston to anupper rest position, and an air vent passage defined by said casing forventing air to the atmosphere, an improved valve assembly mounted insaid casing and comprising a first valve member and a second coaxialvalve member, said valve members being relatively axially moveable, acommunication port between said high pressure chamber and said pressurechamber, said first valve member being moveable from a first position inwhich it sealingly blocks said communication port, and a second positionin which it opens said communication port, said first valve memberdefining a bore therein which is in constant communication with saidpressure chamber in both of said positions, said second valve memberbeing disposed in said bore, and in said first position providingcommunication between said bore and said air vent passage, andcooperable means on said valve members for sealingly preventing suchcommunication in said second position, means for normally maintainingsaid first valve member in said first position whereby said pressurechamber is normally in communication with the atmosphere through saidbore, and means for effecting the movement of said first valve member tosaid second position.
 2. In the pneumatically operable impact tool ofclaim 1 in which said second valve member mounts a ball valve and saidmeans for effecting movement includes a trigger rod for moving said ballvalve from a first ball valve seat to a second ball valve seat.
 3. Inthe pneumatically operable impact tool of claim 2 in which said highpressure air chamber is in constant communication with said ball valvethrough a second bore in said first valve member and the high pressureair maintains said first valve member in said first position when saidball valve is on said first ball valve seat.
 4. In the pneumaticallyoperable impact tool of claim 1 in which said cooperable means on saidvalve members comprises a coaxial seat and a sealing ring which are outof engagement in said first position and which are in sealing engagementto block said bore in said second position.
 5. In the pneumaticallyoperable impact tool of claim 1 in which said valve assembly includes asleeve surrounding said first valve member, said sleeve being fixedagainst axial movement in said casing, said first valve member sealinglyengaging said sleeve and being moveable therein, said first valve memberand said sleeve defining a chamber which is in flow communication withsaid air vent passage and said bore in said first position.
 6. In thepneumatically operable impact tool of claim 1 in which said second valvemember encloses a ball valve and mounts a resiliently loaded trigger rodfor moving said ball valve against said resilient load, and said secondvalve member defines a chamber above said ball valve which communicateswith said high pressure chamber through a second bore defined by saidfirst valve member.
 7. In the pneumatically operable impact tool ofclaim 1 in which said means for normally maintaining said first valvemember in said first position comprises spring means resiliently biasingsaid first valve member to said first position, said biasing meanspreventing driving by the tool when it is first connected to highpressure air.
 8. In a pneumatically operable device for repeatedlyexerting a driving work force in one direction and having a casing, atubular body portion, a sleeve-like cylinder in said body portion and,in combination with the wall of said body portion, defining a narrowfixed volume expansion chamber, a piston sealingly slidable in saidcylinder between retracted and extended positions, a variable volumeexpansion chamber forwardly of the piston, a closure head for the rearend of the cylinder and defining a pressure chamber rearwardly of thepiston, said expansion chambers being in flow communication in allpositions of the piston, valve means selectively operable to admit liveair at working pressure to said pressure chamber to drive the pistonforwardly to its extended position and to relieve such pressure to allowthe piston to return to its retracted position under the influence ofthe expansion of air in said expansion chambers, and one-way check valvemeans automatically effective during forward movement of the piston andafter the piston has assumed its full forward position for admittinglive air at working pressure directly into said expansion chambers fromsaid pressure chamber between said tubular body portion and saidcylinder, said check valve means being effective to seal said expansionchambers against escape of entrapped air to said pressure chamber duringsubstantially the entire return stroke of the piston, the improvementcomprising valve means carried by said piston automatically openable bythe expansion air in said expansion chambers when said piston hasreturned substantially to its rearwardmost retracted position forventing said expansion air through said piston to the pressure chamberand to the atmosphere.
 9. In the pneumatically operable device of claim8 in which said piston defines a passageway in constant communicationwith said expansion chambers and in communication with said valve means,and in which said valve means is biased to a position in which saidpassageway is sealingly closed thereby, said cylinder being relievedadjacent the rearwardmost position of the piston, at which position thevalve means is automatically opened by the expansion air in saidexpansion chambers, thereby to vent said air to atmosphere.
 10. In thepneumatically operable device of claim 9 in which said valve means is anannular gasket and said passageway includes at least one port sealed bysaid annular gasket, said annular gasket being biased to an openposition automatically adjacent the relieved portion of said cylinder.11. In the pneumatically operable device of claim 9 in which said valvemeans is an annular gasket mounted in an annUlar groove defined by saidpiston, said annular gasket being stretched into sealing engagement withthe inner wall of said annular groove, said annular groove defining portmeans communicating with said passageway.